Recently, there has been growing a demand for a battery of a large storage capacity using secondary batteries. Specifically, the demand has been growing in the applications of electric bicycles, electric bikes and electric motorcars, the attention having been focused on batteries of a 100 W to 1000 W. class and also the batteries having an output higher than 1000 W.
The conventional large storage-capacity batteries using secondary batteries have been made up of several lead cells or nickel hydrogen cells in combination, and those having large sizes, low weight and volume densities and also of high costs have been prevalent. For this reason, realizing a large storage-capacity battery having a small size and high weight and volume densities and also of a low cost has been desired.
A high-voltage lithium ion secondary battery, which serves as an elemental cell of a storage battery, has recently been realized, in which a lightweight laminate film is used as a casing. It is expected that the development of a storage battery with use of this lithium ion secondary battery will make it possible to realize a battery of a low cost and a large storage capacity having a small size and high weight and volume densities.
However, even if a storage battery of such a low cost and a large storage capacity having a small size and high weight and volume densities can be realized, many problems are still left to be solved. In particular, when the battery is used for an automobile car, rapid charge/discharge characteristics as well as a high cycle life are required, which gives rise to many problems promptly to be solved such as the lowering of an internal resistance of the battery; a heat-generation problem due to rapid charging; problems in the control of the cell balance in the interior of the battery; and the realization of a highly precise cycle-life predicting circuit.
In order-to solve these problems, it is absolutely necessary to precisely measure the temperature in the interior of the cell. In the conventional secondary battery, it has been common to perform the measurement of the internal temperature by setting a temperature sensor either on a surface layer of the secondary battery or on the positive/negative electrode terminal.
Mounting a temperature sensor on a surface layer of the secondary battery, however, makes it difficult to stack a plurality of secondary batteries when building up a storage battery, because the stack arrangement of flat secondary batteries each with a casing of laminate film has temperature sensors interposed between the secondary batteries, which could result in detecting average temperatures between the stacked secondary batteries, or cause any damage to the secondary battery itself. In some cases, an arrangement has been adopted in which elastic material such as sponge sheets are sandwiched between the secondary batteries, in order to stack secondary batteries avoiding contact with the temperature sensors. The arrangement, however, entails not only lowering of the weight and volume densities but also an increase in the number of processes of constructing the storage battery as well as an increase in component costs.
Attaching a temperature sensor to an electrode terminal, on the other hand, requires an extra long terminal. Consequently, the construction of the storage battery requires a larger volume to accommodate the extra length of terminal, entailing the lowering of the volume density. Furthermore, the heat generated in the electrode terminal by the rapid charge/discharge operation causes the temperature sensor to detect the temperature of the electrode terminal rather than the temperature in the interior of the secondary battery. This has been responsible for the occurrence of the deviations in the life prediction of the secondary battery.
Furthermore, it has been common practice in connecting a cell-balancer circuit or the like to a storage battery to draw out the lead wires for the cell-balancer circuit from charge/discharge electrode terminals of cells when the cells are connected to one another, or to perform the connection between cells through a bus-bar and then draw out the lead wires for the cell-balancer circuit from the bus-bar. As a result, not only the installation of a control system such as a cell-balancer has been troublesome but also the drawing out lead wires for a cell-balancer circuit from the charge/discharge electrode terminals has prevented the electrode terminals from being shortened, entailing difficulty in lowering the internal resistance of the storage battery.